SUMMARY
Each time I try to open any of PDF files with invoice from particular issuer the Ocular process crashes. It's 100% reproducible and happens on multiple files from the same source. I didn't observe any adverse behavior with pdf files from other sources.

STEPS TO REPRODUCE
1. Try to open the problematic pdf either by clicking on it, dragging into Ocular o via File Open menu.
2. Since the PDF files causing the crash contain somewhat sensitive information I can share them privately with developer when contacted.

ADDITIONAL INFORMATION

-- Backtrace:
Application: Okular (okular), signal: Aborted

[KCrash Handler]
#4  __pthread_kill_implementation (no_tid=0, signo=6, threadid=140291996588480) at ./nptl/pthread_kill.c:44
#5  __pthread_kill_internal (signo=6, threadid=140291996588480) at ./nptl/pthread_kill.c:78
#6  __GI___pthread_kill (threadid=140291996588480, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
#7  0x00007f984aa27476 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#8  0x00007f984aa0d7f3 in __GI_abort () at ./stdlib/abort.c:79
#9  0x00007f984acafbbe in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#10 0x00007f984acbb24c in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#11 0x00007f984acbb2b7 in std::terminate() () from /lib/x86_64-linux-gnu/libstdc++.so.6
#12 0x00007f984acbb518 in __cxa_throw () from /lib/x86_64-linux-gnu/libstdc++.so.6
#13 0x00007f984acb238a in std::__throw_logic_error(char const*) () from /lib/x86_64-linux-gnu/libstdc++.so.6
#14 0x00007f983c2d652e in SignatureHandler::getSignerName[abi:cxx11]() () from /lib/x86_64-linux-gnu/libpoppler.so.118
#15 0x00007f983c1cec12 in FormFieldSignature::validateSignature(bool, bool, long, bool, bool) () from /lib/x86_64-linux-gnu/libpoppler.so.118
#16 0x00007f983d715896 in Poppler::FormFieldSignature::validate(int, QDateTime const&) const () from /lib/x86_64-linux-gnu/libpoppler-qt5.so.1
#17 0x00007f983d715d14 in Poppler::FormFieldSignature::validate(Poppler::FormFieldSignature::ValidateOptions) const () from /lib/x86_64-linux-gnu/libpoppler-qt5.so.1
#18 0x00007f983d79512b in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okular/generators/okularGenerator_poppler.so
#19 0x00007f983d799e64 in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okular/generators/okularGenerator_poppler.so
#20 0x00007f983d78b392 in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okular/generators/okularGenerator_poppler.so
#21 0x00007f98443f20d5 in ?? () from /lib/x86_64-linux-gnu/libOkular5Core.so.9
#22 0x00007f98443fea73 in Okular::Document::openDocument(QString const&, QUrl const&, QMimeType const&, QString const&) () from /lib/x86_64-linux-gnu/libOkular5Core.so.9
#23 0x00007f98445415a5 in Okular::Part::doOpenFile(QMimeType const&, QString const&, bool*) () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okularpart.so
#24 0x00007f984454384b in Okular::Part::openFile() () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okularpart.so
#25 0x00007f984c847325 in ?? () from /lib/x86_64-linux-gnu/libKF5Parts.so.5
#26 0x00007f984c84bba7 in KParts::ReadOnlyPart::openUrl(QUrl const&) () from /lib/x86_64-linux-gnu/libKF5Parts.so.5
#27 0x00007f98445421db in Okular::Part::openUrl(QUrl const&, bool) () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okularpart.so
#28 0x0000558f069227bb in ?? ()
#29 0x0000558f06927a32 in ?? ()
#30 0x0000558f0691ee11 in ?? ()
#31 0x00007f984aa0ed90 in __libc_start_call_main (main=main@entry=0x558f0691d700, argc=argc@entry=2, argv=argv@entry=0x7ffc5f70cac8) at ../sysdeps/nptl/libc_start_call_main.h:58
#32 0x00007f984aa0ee40 in __libc_start_main_impl (main=0x558f0691d700, argc=2, argv=0x7ffc5f70cac8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffc5f70cab8) at ../csu/libc-start.c:392
#33 0x0000558f0691f675 in ?? ()
[Inferior 1 (process 284309) detached]